Computer tomograph comprising energy discriminating detectors

ABSTRACT

The invention relates to a computer tomograph comprising a detector unit ( 2 ) consisting of a plurality of detectors ( 1 ) for identifying X-ray radiation ( 40 ). According to the invention, the individual detectors ( 1 ) of the detector unit ( 2 ) are configured to receive incident quanta of the X-ray radiation ( 40 ) and to record the received X-ray radiation ( 40 ), both in terms of its intensity and in terms of the quantum energy of the individual X-ray quanta of the received X-ray radiation ( 40 ). The invention also relates to a corresponding method for identifying X-ray radiation by means of a computer tomograph that comprises a detector unit ( 2 ) consisting of a plurality of detectors ( 1 ).

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/DE03/00818 which has an Internationalfiling date of Mar. 13, 2003, which designated the United States ofAmerica and which claims priority on German Patent Application number DE102 12 638.0 filed Mar. 21, 2002, the entire contents of which arehereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention generally relates to a computer tomograph and to amethod for verification of X-ray radiation by use of a detector unitwhich includes a large number of detectors.

BACKGROUND OF THE INVENTION

Examinations are carried out with the aid of computer tomographs in manymedical problem situations. Examinations such as these are also carriedout for test purposes in a number of machine construction fields, inparticular in materials science and flight safety.

X-ray radiation is used in this case because these solid bodies, forexample non-metallic bodies, can be partially penetrated, so that it ispossible to obtain knowledge about the distribution of materials withinthe body being analyzed.

The use of X-ray radiation has the disadvantage that, beyond a certaindose, it can damage biological tissue. Therefore, particularly inmedicine, it is desirable to keep the radiation dose required for ameasurement low.

In order to verify X-ray radiation, it is known that this radiation canbe absorbed by specific scintillation materials, with the energy in theabsorbed X-ray quanta being converted to light. The number of photonsproduced per X-ray quantum is in this case generally approximatelyproportional to their quantum energy.

A photodiode converts the light to a current which is digitized by ananalog/digital converter. Since the self-absorption of the light in thescintillation material frequently has molecules added to it, which causethe frequency of the light that is produced to be shifted, in order inthis way to prevent self-absorption of the light that is produced.

Furthermore, specific semiconductor materials in which the incidentX-ray radiation can produce charge carriers, are also known forverification of X-ray radiation. The number of charge carriers producedper X-ray quantum is in this case generally approximately proportionalto their quantum energy.

The known detectors for verification of X-ray radiation make use of theeffects described above. In this case, it should be noted that the knownintegrating detectors produce only one measured value per measurement.The light flashes or charges which are produced by the large number ofX-ray quanta received in each measurement period are thus integrated upover the duration of the measurement period. The intensity of thereceived X-ray radiation (the number of received X-ray quanta of mediumquantum energy per unit time) is then obtained by division of the valueintegrated up by the detector by the medium quantum energy per X-rayquantum, which can be determined stochastically.

Since the measurement X-ray radiation emitted for measurement purposesin computer tomography normally has a polychromatic spectrum, hardeningeffects must be taken into account in this context. When the measurementX-ray radiation emitted from a radiation source passes through ameasurement object, the X-ray radiation is subject in some cases tosevere suppression of low-energy components of its spectrum, dependingon the materials it is passing through and on the length of the beampath through the materials. The scattered radiation is thus shifted inthe same way as the medium quantum energy of the received X-ray quantatoward higher energies in the spectrum.

In order to verify the two-dimensional distribution and thus to createan image of the incident X-ray radiation, it is known for a large numberof identical detectors to be combined to form a detector unit fordetection of incident radiation, and for emission of corresponding imageinformation. The detectors are in this case preferably arrangedalongside one another on a plane, in the form of an array.

This results in a different value for the actual medium quantum energyper X-ray quantum, as a result of hardening effects for each detector ina detector unit as a function of the material distribution in themeasurement object being analyzed. This actual value can be determinedonly approximately by means of stochastic methods. Particularly in areasin which different materials in the measurement object being analyzedare adjacent to one another (for example bone edges), the approximatedetermination of the medium quantum energy per X-ray quantum is subjectto major errors, despite numerical corrections.

A further disturbance variable on the measurement of X-ray radiation byuse of computer tomographs is the scattered radiation, which ispronounced to a greater or lesser extent depending on the measurementobject being analyzed. The scattered radiation may make up several tensof percentage points of the emitted measurement X-ray radiation,depending on the spectrum of the emitted measurement X-ray radiation andon the nature of the measurement object being analyzed. This leads to aconsiderable deterioration in the contrast in the measurement resultobtained from the detectors in the detector unit.

A scattered radiation grid, through which only X-ray quanta which are ina specific direction and which have a specific energy (and which arethus important for the measurement) can pass, is therefore providedupstream of the detector unit in known computer tomographs. Thescattered radiation grid generally has a specific collimator system inthe form of a lamella arrangement, so that X-ray quanta of the emittedmeasurement X-ray radiation which strike the lamella walls are alsoabsorbed.

The provision of a scattered radiation grid accordingly means that acertain percentage of the radiation quanta of measurement X-rayradiation which is emitted for measurement purposes is absorbed in thescattered radiation grid, and can thus no longer be detected by thedetectors.

In consequence, the intensity of the radiation emitted for measurementpurposes must be increased appropriately, owing to the scatteredradiation grid.

In medical applications, this unavoidably leads to an increased patientdose. Furthermore, the scattered radiation can frequently also not besufficiently well suppressed by the provision of a scattered radiationgrid.

SUMMARY OF THE INVENTION

An object of an embodiment of the present invention is to provide acomputer tomograph and a method for verification of X-ray radiation byway of a detector unit which includes large number of detectors and inwhich an adverse effect on the measurement result caused by scatteredradiation quanta or hardening effects is easily and reliably avoided.

The object may be achieved by a computer tomograph having:

a radiation source for emission of X-ray radiation with a predeterminedintensity and a predetermined spectrum;

a detector unit, which comprises a large number of detectors, forverification of X-ray radiation, with the individual detectors in thedetector unit being designed to receive incident X-ray quanta in theX-ray radiation and to detect the number of X-ray quanta in the receivedX-ray radiation whose quantum energy exceeds a predetermined thresholdvalue;

a transmission device for transmission of the information detected bythe detectors in the detector unit to an evaluation device and

an evaluation device which is designed to calculate a measurement resultfrom a measurement object through which the X-ray radiation has passedon the basis of the information detected by the detectors in thedetector unit, with the individual detectors in the detector unit beingdesigned to detect both the intensity and the quantum energy of theindividual detectors equipped to receive impinging X-ray quanta in thereceived X-ray radiation and, for each measurement period, to emit aspectrum which, in addition to information about the number of X-rayquanta of medium quantum energy received in each measurement period, andhence the intensity, also contains information about the respectivequantum energy of the X-ray quanta, and thus the spectrum of thereceived X-ray radiation; and in that the evaluation device is alsodesigned to calculate the measurement result from the measurement objecton the basis of the information detected by the detectors relating tothe intensity and quantum energy of the individual X-ray quanta in thereceived X-ray radiation, taking into account the intensity and thespectrum of the X-ray radiation emitted from the radiation source.

Since the detectors in the detector unit in the computer tomographaccording to an embodiment of the present invention are designed toreceive incident X-ray radiation and to detect the intensity and thequantum energy of the individual X-ray quanta in the received X-rayradiation, a spectrum is emitted at the output of the detectors in thedetector unit, instead of a single measured value per measurementperiod, which spectrum contains not only information about the number ofX-ray quanta of medium quantum energy (intensity) received permeasurement period, but also information about the respective quantumenergy of the X-ray quanta (the spectrum) in the received X-rayradiation.

A design such as this allows a particularly detailed measurement resultto be calculated in a particularly simple and reliable manner from ameasurement object being analyzed by comparison of the intensity and ofthe spectrum of the X-ray radiation emitted from a radiation source withthe intensity detected by the detectors in the detector unit for thecomputer tomograph according to an embodiment of the invention and thespectrum of the received X-ray radiation.

The information obtained in this way can be used to further suppressinfluences caused by scattered radiation, in addition to a scatteredradiation grid which may be provided.

Furthermore, by analysis of the spectrum obtained, it is possible toreliably to detect hardening effects in the received X-ray radiation,such as those which occur at bone edges, on the basis of the shift inthe spectrum of the received X-ray radiation. The hardening effectsdetected in this way can then be taken into account, and possiblycorrected, appropriately in the further processing of the informationobtained from the detectors in the detector unit.

Furthermore, during the further processing of the information obtainedfrom the detectors in the detector unit, it is advantageous that aquantitative evaluation of the spectral data obtained with the inventivecomputer tomographs (for example by ρ-Z transformation) is possible withthe methods known for conventional computer tomographs.

Furthermore, the electronics for the detectors for the computertomograph according to an embodiment of the invention have considerablyless analog parts than the electronics of conventional detectors, sincethere is no need to integrate up a large number of result elementsproduced by X-ray quanta in the received X-ray radiation. Theelectronics for the computer tomograph according to the invention maythus be smaller, cheaper and more resistant to disturbances.

In summary, according to an embodiment of the present invention, it ispossible to produce a computer tomograph having a detector unit, whichincludes a large number of detectors, for verification of X-rayradiation, in which the adverse effect on the measurement result causedby scattered radiation quanta or hardening effects is simply andreliably avoided.

According to a first preferred embodiment, the detectors in the detectorunit have a large number of parallel-connected comparators, each havinga threshold value, and each comparator has an associated counter, andthe comparators are designed to increment the respectively associatedcounter by one unit when the quantum energy of an X-ray quantum in thereceived X-ray radiation exceeds the threshold value of the respectivecomparator.

A detector design such as this makes it possible to detect both theintensity and the spectrum of the received X-ray radiation in aparticularly simple manner. Furthermore, since the number of receivedX-ray quanta with a specific quantum energy are also detected by all thecounters in the comparators with lower threshold values, no results arerejected. The number of X-ray quanta with a quantum energy within athreshold value range can then easily be calculated from the differencebetween the counts of two comparators with adjacent threshold values. Inthis case, use is made of the correlation of the counting rates ofcounters, so that the statistical error does not rise during thesubtraction process.

The threshold values of the comparators are preferably freely variable,so that the computer tomograph according to an embodiment of theinvention can be matched to different measurement objects to beanalyzed, and to different measurement methods.

The information obtained from the detectors in the detector unit can beprocessed further particularly easily by the detectors in the detectorunit having a large number of pulse logic devices. The pulse logicdevices provide time normalization of the output signals from thecomparators. In this case, one pulse logic device is in each caseconnected downstream from each of the comparators, and is connectedupstream of each of the counters.

The detectors in the detector unit preferably have a receiving area forthe X-ray radiation, which receiving area is formed fromgadoliniumoxysulfide ceramic, bismuth germanium oxide or lutetiumoxyorthosilicate. These very fast scintillator materials allow acounting rate, which is preferably used in the computer tomographaccording to the invention, of up to 10 MHz for pixel sizes of about1/50 mm².

Alternatively, however, the detectors may also have a direct-conversionreceiving area for receiving the X-ray radiation, which receiving areais preferably formed from cadmium zinc telluride or cadmium telluride.

The advantage of direct-conversion detectors is, in particular, that alarge proportion of the evaluation electronics which are required forfurther processing of a signal produced by the detector can beintegrated in the detectors, thus making it possible to reduce thecomplexity of the detector unit, not least as a result of the reductionin the number of lines to be passed out.

An object on which an embodiment of the present invention is based isalso achieved by a method for verification of X-ray radiation by way ofa computer tomograph which has a detector unit comprising a large numberof detectors, having the following steps:

detection of the number of X-ray quanta whose quantum energy exceeds apredetermined threshold value in the X-ray radiation received by meansof the individual detectors in the detector unit;

transmission of the information obtained by means of the detectors inthe detector unit to an evaluation device and

calculation of a measurement result from a measurement object throughwhich the X-ray radiation has passed, by means of the evaluation deviceon the basis of the information detected by the detectors in thedetector unit, with both the intensity and the quantum energy of theindividual X-ray quanta in the X-ray radiation received by means of theindividual detectors in the detector unit being detected, and, in that,in each measurement period, the individual detectors in the detectorunit emit a spectrum which, in addition to information about the numberof X-ray quanta of medium quantum energy received in each measurementperiod, and hence the intensity, also contains information about therespective quantum energy of the X-ray quanta, and thus the spectrum ofthe received X-ray radiation, and in that the measurement result fromthe measurement object is calculated by means of the evaluation deviceon the basis of the information detected by the detectors relating tothe intensity and quantum energy of the individual X-ray quanta in thereceived X-ray radiation, taking into account the intensity and thespectrum of the X-ray radiation emitted from a radiation source.

A method such as this makes it possible to correct for scatteredradiation influences and hardening effects in a particularly simple andreliable manner by comparison of the intensity and of the spectrum ofthe X-ray radiation which is emitted from a radiation source with theintensity, as detected by the detectors in the detector unit, and thespectrum of the received X-ray radiation, and thus to calculate aparticularly detailed measurement result from a measurement object beinganalyzed.

With a design such as this, by comparison of the intensity and of thespectrum of the X-rays emitted from a radiation source with theintensity and spectrum of the X-rays received by the detectors of thedetector unit of the computer tomograph according to the invention andthe spectrum of the received X-ray radiation, it is possible tocalculate a particularly detailed measurement result from a measurementobject being analyzed, in a particularly simple and reliable manner.

According to a first embodiment of a method of the invention, the stepof detection of the X-ray quanta which are received by way of thedetector in the detector unit comprises the following steps:

detection of a signal which is produced in the detector as a consequenceof a received X-ray quantum, hose signal level is proportional to thequantum energy in the received X-ray quantum;

comparison of the signal level with a large number of predeterminedthreshold values;

incrementation of a counter, which is in each case associated with onerange between two adjacent threshold values, by one unit when the signallevel of the signal is in the range between the two adjacent thresholdvalues.

Since the counts of the counters thus receive both information about thenumber of X-ray quanta received and about the respective quantum energyof the received X-ray quanta at the end of a measurement period, it issimple to indicate both the intensity and the spectrum of the receivedX-ray radiation on the basis of the counts of the counters.

According to an alternative, second, particularly preferred embodimentof the method according to the invention, the step of detection of theX-ray quanta which are received by way of the detector in the detectorunit comprises the following steps:

detection of a signal which is produced in the detector as a consequenceof a received X-ray quantum, whose signal level is proportional to thequantum energy in the received X-ray quantum;

comparison of the signal level with a large number of predeterminedthreshold values;

incrementation of counters, which are each associated with one thresholdvalue, by one unit when the signal level of the signal exceeds therespective threshold value.

One particularly advantageous feature of this procedure is that noresults are rejected since the number of received X-ray quanta with aspecific quantum energy are also detected by all the counters with alower threshold value. The number of X-ray quanta with a quantum energywithin a threshold value range can then easily be calculated from thedifference between the counts of the counters of two comparators withadjacent threshold values.

It is also advantageous for a signal which is produced in the detectoras a consequence of a received X-ray quantum to be rejected if thedetermined signal level of the signal is lower than a lowest thresholdvalue.

Another particularly advantageous feature of the method according to theinvention is for the threshold values to be freely variable.

According to one particularly preferred embodiment, the method accordingto the invention also comprises the following steps:

transmission of the information obtained by means of the detectors to anevaluation device;

calculation of a measurement result from a measurement object throughwhich the X-ray radiation has passed, by means of the evaluation deviceon the basis of the information detected by the detectors and takinginto account the intensity and the spectrum of the X-ray radiationemitted from a radiation source.

A method such as this makes it possible to correct for scatteredradiation influences and hardening effects in a particularly simple andreliable manner by comparison of the intensity and of the spectrum ofthe X-ray radiation which is emitted from a radiation source with theintensity, as detected by the detectors in the detector unit, and thespectrum of the received X-ray radiation, and thus to calculate aparticularly detailed measurement result from a measurement object beinganalyzed.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages, features and details of the invention will becomeevident from the description of illustrated exemplary embodiments givenhereinbelow and the accompanying drawings, which are given by way ofillustration only and thus are not limitative of the present invention,wherein identical elements are provided with the same reference symbolsand, wherein:

FIG. 1 shows a detector unit, which includes a large number ofdetectors, for a computer tomograph for verification of X-ray radiation,

FIG. 2 shows, schematically, major elements of a detector for thecomputer tomograph according to the invention, based on one particularlypreferred embodiment,

FIG. 3 shows the principle of a development of the detector shown inFIG. 2,

FIG. 4 shows, schematically, major elements of one preferred measurementlayout with the computer tomograph according to an embodiment of theinvention, and

FIG. 5 shows a flowchart of one particularly preferred embodiment of themethod according to the invention for verification of X-ray radiation,by means of a computer tomograph which has a detector unit comprising alarge number of detectors.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a detector unit, which includes a large number ofdetectors, for a computer tomograph for verification of X-ray radiation.

The individual detectors 1 in the detector unit 2 are each designed tobe the same, and each have a receiving area 4 for X-ray radiation.

In the illustrated preferred embodiment, the receiving areas 4 of thedetectors have a scintillator material, in which impinging X-ray quantaare converted to light. In this case, the number of photons which areproduced by a received X-ray quantum is approximately proportional tothe quantum energy of the received X-ray quantum. In FIG. 1, bismuthgermanium oxide (Bi₄Ge₃O₁₂) is used as the scintillator material.However, alternatively, gadoliniumoxysulfide (Gd₂O₂S) ceramic orlutetium oxyorthosilicate (Lu₂SiO₅) are also highly suitable owing tothe speed of these scintillator materials.

However, alternatively, the receiving areas 3 of the detectors may alsobe formed from cadmium zinc telluride (CdZnTe) or cadmium telluride(CdTe), since these materials can emit an electrical signal directly inresponse to a received X-ray quantum (that is to say without having topass through light). The value/the level of the signal (in the form of acharge, a voltage or a current that is produced) is in this caseapproximately proportional to the quantum energy in the received X-rayquantum. Direct-conversion detectors have the particular advantage thatsome of the evaluation electronics which are used for further processing(but which are not shown) for the detectors can be integrated directlyin the respective detector.

FIG. 2 shows, schematically, major elements of a detector for thecomputer tomograph according to the invention, based on one particularlypreferred embodiment.

As explained above, a received X-ray quantum produces a signal in thereceiving area 3 of the detector 1 illustrated in FIG. 2, whose signallevel is proportional to the quantum energy of the received X-rayquantum. This signal is amplified by an amplifier 12.

A detection circuit 16, which has three parallel-connected comparators131, 132 and 133, is connected to the amplifier 12. Each of theparallel-connected comparators 131, 132 and 133 is assigned a differentfreely variable threshold value. In the illustrated example, thecomparator 131 is assigned the lowest threshold value, and thecomparator 133 is assigned the highest threshold value.

The comparators 131, 132 and 133 are designed in order to compare thesignal emitted from the amplifier 12 with their respective thresholdvalue, and to emit a positive signal if the signal received from theamplifier 12 is higher than the respective threshold value.

A pulse logic device 141, 142 and 143 is in each case connected inseries with the comparators 131, 132 and 133. Each of the pulse logicdevices 141, 142 and 143 is designed to provide time normalization ofthe output signals from the comparators 131, 132 and 133. A counter 151,152 and 153 is in each case connected in series with the pulse logicdevice 141, 142 and 143.

A positive signal emitted from the respective comparator 131, 132 and133 and normalized by the respective pulse logic device 141, 142 and 143results in the respective counter 151, 152 and 153 being incremented byone unit.

In this case, the pulse logic devices 141, 142 and 143 are preferablysynchronized to one another, and have a common control line, which isnot shown.

When, in the particularly preferred embodiment of a detector for thecomputer tomograph according to an embodiment of the invention as shownin FIG. 2, an X-ray quantum is thus received, whose quantum energy ishigher than the threshold value of the comparator 132 and is thus alsohigher than the threshold value of the comparator 131, but is lower thanthe threshold value of the comparator 133, then both the comparator 131and the comparator 132 emit a positive output signal. As a result ofthis, the counters 151 and 152 are incremented upward by one unit. Thecomparator 133 on the other hand emits a negative output signal, and thecounter 153 associated with the comparator 133 remains unchanged.

When, in the illustrated example, an X-ray quantum is received whosequantum energy is higher than the threshold value of the comparator 131but is lower than the threshold value of the comparator 132 and is thusalso lower than the threshold value of the comparator 133, then, in acorresponding manner, only the counter 151 is incremented by one, while,in contrast, the counters 152 and 153 remain unchanged.

If, in contrast, an X-ray quantum is received whose quantum energy islower than the threshold value of the comparator 131, then the X-rayquantum is not detected by any of the counters 151, 152 or 153.

It is thus possible by skilful choice of the lowest threshold value toexclude from the start scattered radiation inputs, since they are notdetected by any of the counters.

As will be clearly evident from the examples described above, the numberof received X-ray quanta whose quantum energy corresponds to arespective threshold value range can easily be calculated by thedifference between the counts of the counters in comparators foradjacent threshold values.

The particularly preferred embodiment which is illustrated in FIG. 2allows, owing to better clarity, only a spectral distinction betweenX-ray quanta in four quantum energy ranges (lower than the thresholdvalue of the comparator 131, between the threshold values of thecomparators 131 and 132, between the threshold values of the comparators132 and 133, and higher than the threshold value of the comparator 133).

In order on this basis to achieve a higher spectral resolution for thereceived X-ray radiation, as is desirable in practice, all that isnecessary is to provide in the detection circuit 16, a higher number ofparallel switched comparators with different threshold values. Asindicated in FIG. 3, a pulse logic device and a counter can once againbe associated with each comparator. In consequence, a virtuallyindefinitely fine spectral resolution of the X-ray radiation received bythe receiving area 3 of the detector 1 can thus be achieved in a simplemanner.

The detector 1, as described above, in the detector unit 2 for thecomputer tomograph according to an embodiment of the invention thusdetects both the intensity and the quantum energy of the individualX-ray quanta in the received X-ray radiation.

According to one particularly preferred embodiment, which is illustratedin FIG. 4, it is also particularly advantageous for the computertomograph according to an embodiment of the invention to have, inaddition to a radiation source 41 for emission of X-ray radiation 40with a predetermined intensity and a predetermined spectrum, atransmission device 43 for transmission of the information detected bythe detectors 1 in the detector unit 2 to an evaluation device 44.

In this case, the evaluation device 44 is preferably designed to use theinformation detected by the detectors 1 in the detector unit 2 tocalculate a measurement result from a measurement object 42 throughwhich the X-ray radiation 40 from the radiation source 41 has passed,taking into account the intensity and the spectrum of the X-rayradiation 40 emitted from the radiation source 41.

This design makes it possible to obtain a particularly accurate anderror-free measurement result, since scattered radiation influences aswell as hardening effects can be effectively detected, quantified, andthus also corrected.

In the following text, with reference to FIG. 5 and using a flow chart,a preferred embodiment of the method according to an embodiment of theinvention is described, for verification of X-ray radiation by way of acomputer tomograph which has a detector unit 2 including a large numberof detectors 1.

According to an embodiment of the invention, in the case of the method,both the intensity and the quantum energy of a single X-ray quantum inthe received X-ray radiation 40 received by way of one detector 1 in thedetector device 2 are detected.

According to the preferred embodiment illustrated in FIG. 5, the step ofdetection of the X-ray quanta received by way of a respective detector 1in the detector unit 2 has the following steps:

In a first step S1, the detector 1 is continuously monitored forincident X-ray quanta, in order to detect an analog signal emitted fromthe detector 1 as a consequence of an X-ray quantum having beenreceived. In this case, the detector 1 is designed such that the value(the level) of the emitted signal is proportional to the quantum energyof the received X-ray quantum (as is the case, by way of example, withscintillation detectors). This emitted signal may, for example, be anelectric current, a voltage or a charge with a specific magnitude.

In step S2, if a signal which is produced as a consequence of an X-rayquantum being received by the detector 1 is detected, then the value ofthe signal that is produced is first of all compared with a first,lowest threshold value in order to determine the quantum energy of thereceived X-ray quantum, causing the signal, in step S3.

If a decision is made in step S4 that the value of the signal is higherthan the lowest threshold value, then a counter 151 which is associatedwith the lowest threshold value is incremented by one unit in the nextstep S5.

Otherwise, the method returns to step S1, in which the detector 1carries out continuous monitoring for incident X-ray quanta.

If the decision as made in the step S4 that the value of the signal ishigher than the lowest threshold value, then the signal, then, afterincrementing the counter 151 (see step S5) that is associated with thelowest threshold value, the signal is compared with the next-higherthreshold value, in step S6.

If the decision is made in the following step S7 that the value of thesignal is higher than this next-higher threshold value, then the counter152, 153 which is associated with this threshold value is alsoincremented in the step S8.

The signal is then once again compared with the respective next-higherthreshold value in step S6.

If the decision is made in step S7 that the signal value is lower thanthe respective threshold value, then the method returns to step S1, inwhich the detector 1 carries out continuous monitoring for incidentX-ray quanta.

It should be noted that individual steps in the method described inconjunction with FIG. 5 (in particular the steps S3, S4, S5 and S6, S7,S8), if they are carried out by the electronic detection circuit asshown in FIG. 2, are preferably not processed in serial form, as shownin FIG. 5, but in parallel form. In this case, clocking of the steps inFIG. 2 is predetermined by the pulse logic device, and is preferablyseveral MHz.

As is obvious from the embodiment according to the invention, explainedwith reference to FIG. 5, that a signal generated by the detector 1, asa consequence of a received X-ray quantum is rejected if the signalvalue is lower than the lowest threshold value. It is thus possible tolargely exclude scattered radiation influences by a suitable choice ofthe lowest threshold value.

However, in principle, the threshold values are in this case freelyvariable, so that it is even feasible to use a lowest threshold value ofzero, or close to zero. A low threshold value such as this has theadvantage that no event is rejected.

Once the method described above has been carried out, the number ofincident X-ray quanta with a quantum energy which corresponds to aspecific threshold range can easily be determined by the differencebetween the counts of the counters which are associated with adjacentthreshold values.

In the described particularly preferred embodiment, the methodillustrated in FIG. 5 is carried out in a detection circuit 16 which isintegrated in each detector 1 in the detector unit 2 for the computertomograph according to an embodiment of the invention.

According to one alternative embodiment of the method according to theinvention, which is not specifically illustrated, it is also possible,in contrast to the embodiment described above, to in each case incrementby one unit only that counter which is in each case associated with arange between two adjacent threshold values, while, in contrast, theother counters remain constant. This allows the number of incident X-rayquanta with a quantum energy associated with a specific threshold rangeto be emitted directly without any further calculations. Thisalternative embodiment of the method according to the invention can beimplemented in circuitry particularly easily in that an AND gate withinverting input is connected upstream. In this case, the outputs of thecomparators for adjacent threshold values can be connected (possibly viaa pulse logic device) to the inputs of this AND gate.

In the method according to an embodiment of the invention, it is alsoparticularly advantageous for the method also to have the steps oftransmission of the information obtained by way of the detectors 1 to anevaluation device 44, and calculation of a measurement result from ameasurement object 42 through which the X-ray radiation 40 has passed byway of the evaluation device 44. In this case, the measurement result iscalculated by the evaluation device 44 on the basis of the informationdetected by the detectors 1, taking into account the intensity and thespectrum of the X-ray radiation 40 emitted from a radiation source 41.During the calculation of the measurement result for the measurementobject 42, it is thus possible to correct not only for the scatteredradiation influences but also for hardening influences, with a highdegree of error confidence.

In summary, according to an embodiment of the present invention, thedetection of both the intensity and the spectrum of X-ray radiation 40which is received by way of a detector 1 in a detector unit 2 for acomputer tomograph allows a computer tomograph and a method to beprovided for verification of X-ray radiation 40 by way of a detectorunit 2 which includes a large number of detectors 1, in which anyadverse effect on the measurement result caused by scattered radiationquanta or hardening effects can easily and reliably be avoided.

Exemplary embodiments being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the present invention, andall such modifications as would be obvious to one skilled in the art areintended to be included within the scope of the following claims.

1. A computer tomograph, comprising: a radiation source for emission ofX-ray radiation with a predetermined intensity and a predeterminedspectrum; a detector unit, including a plurality of detectors, forverification of X-ray radiation, wherein individual detectors of thedetector unit are designed to receive incident X-ray quanta in the X-rayradiation and to detect the number of X-ray quanta in the received X-rayradiation whose quantum energy exceeds a predetermined threshold value;a transmission device for transmission of the information detected bythe detectors in the detector unit; and an evaluation device, designedto calculate a measurement result from a measurement object throughwhich the X-ray radiation has passed on the basis of the informationdetected by the detectors in the detector unit, wherein the individualdetectors in the detector unit are designed to detect both the intensityand the quantum energy of the individual X-ray quanta in the receivedX-ray radiation, and, for each measurement period, to emit a spectrumwhich, in addition to information about the number of X-ray quanta ofmedium quantum energy received in each measurement period, and hence theintensity, also contains information about the respective quantum energyin the X-ray quanta, and thus the spectrum of the received X-rayradiation; wherein the evaluation device is also designed to calculatethe measurement result from the measurement object on the basis of theinformation detected by the detectors relating to the intensity andquantum energy of the individual X-ray quanta in the received X-rayradiation, taking into account the intensity and the spectrum of theX-ray radiation emitted from the radiation source; wherein the detectorsin the detector unit include a plurality of parallel-connectedcomparators, each having a threshold value and each including anassociated counter, the comparators being designed to increment therespectively associated counter by one unit when the quantum energy ofan X-ray quantum in the received X-ray radiation exceeds the thresholdvalue of the respective comparator; and wherein the detectors in thedetector unit include a plurality of pulse logic devices, one pulselogic device being connected downstream from the respective comparatorsand upstream of the respective counters, the pulse logic devicesproviding time normalization of the output signals from the comparators.2. The computer tomograph as claimed in claim 1, wherein the thresholdvalues of the comparators are freely variable.
 3. The computer tomographas claimed in claim 1, wherein the detectors in the detector unitinclude a receiving area for the X-ray radiation, the receiving areabeing formed from at least one of gadoliniumoxysulfide ceramic, bismuthgermanium oxide and lutetium oxyorthosilicate.
 4. The computer tomographas claimed in claim 1, wherein the detectors in the detector unitinclude a direct-conversion receiving area for the X-ray radiation, thereceiving area being formed from at least one of cadmium zinc tellurideand cadmium telluride.
 5. A method for verification of X-ray radiationby way of a computer tomograph which has a detector unit including aplurality of detectors, the method comprising: detecting a number ofX-ray quanta whose quantum energy exceeds a predetermined thresholdvalue of the X-ray radiation received, using the individual detectors inthe detector unit; transmitting the information detected; andcalculating a measurement result from a measurement object through whichthe X-ray radiation has passed, on the basis of the information detectedby the detectors, wherein both the intensity and the quantum energy ofthe individual X-ray quanta in the X-ray radiation received by theindividual detectors in the detector unit are detected, wherein theindividual detectors in the detector unit emit, for each measurementperiod, a spectrum which, in addition to information about the number ofX-ray quanta of medium quantum energy received in each measurementperiod, and hence the intensity, also contains information about therespective quantum energy of the X-ray quanta, and thus the spectrum ofthe received X-ray radiation, wherein the measurement result from themeasurement object is calculated on the basis of the informationdetected by the detectors relating to the intensity and quantum energyof the individual X-ray quanta in the received X-ray radiation, takinginto account the intensity and the spectrum of the X-ray radiationemitted from a radiation source, and wherein the detectors in thedetector unit include a plurality of parallel-connected comparators,each having a threshold value and each including an associated counter,the comparators being designed to increment the respectively associatedcounter by one unit when the quantum energy of an X-ray quantum in thereceived X-ray radiation exceeds the threshold value of the respectivecomparator, and wherein the detectors in the detector unit include aplurality of pulse logic devices, one pulse logic device being connecteddownstream from the respective comparators and upstream of therespective counters, the pulse logic devices providing timenormalization of the output signals from the comparators.
 6. The methodfor verification of radiation as claimed in claim 5, wherein thedetection of the X-ray quanta which are received by way of the detectorin the detector unit comprises: detecting a signal, produced in thedetector, as a consequence of a received X-ray quantum, whose signallevel is proportional to the quantum energy in the received X-rayquantum; comparing the signal level with a large number of predeterminedthreshold values; and incrementing a counter, which is in each caseassociated with one range between two adjacent threshold values, by oneunit when the signal level of the signal is in the range between the twoadjacent threshold values.
 7. The method for verification of radiationas claimed in claim 6, wherein a signal, which is produced in thedetector as a consequence of a received X-ray quantum, is rejected ifthe determined signal level of the signal is lower than a lowestthreshold value.
 8. The method for verification of radiation as claimedin claim 7, wherein the threshold values are freely variable.
 9. Themethod for verification of radiation as claimed in claim 6, wherein thethreshold values are freely variable.
 10. The method for verification ofradiation as claimed in claim 5, wherein the detection of the X-rayquanta which are received by use of the detector in the detector unitcomprises: detecting a signal which is produced in the detector as aconsequence of a received X-ray quantum, whose signal level isproportional to the quantum energy in the received X-ray quantum;comparing the signal level with a large number of predeterminedthreshold values; and incrementing counters, which are each associatedwith one threshold value, by one unit when the signal level of thesignal exceeds the respective threshold value.
 11. The method forverification of radiation as claimed in claim 10, wherein a signal,which is produced in the detector as a consequence of a received X-rayquantum, is rejected if the determined signal level of the signal islower than a lowest threshold value.
 12. The method for verification ofradiation as claimed in claim 11, wherein the threshold values arefreely variable.
 13. The method for verification of radiation as claimedin claim 10, wherein the threshold values are freely variable.
 14. Anapparatus for verification of X-ray radiation using a computertomograph, comprising: means, including a plurality of individualdetectors, for detecting a number of X-ray quanta whose quantum energyexceeds a predetermined threshold value of the X-ray radiation received;means for transmitting the information detected; and means forcalculating a measurement result from a measurement object through whichthe X-ray radiation has passed, on the basis of the informationdetected, wherein both the intensity and the quantum energy of theindividual X-ray quanta in the X-ray radiation received by theindividual detectors are detected, wherein the individual detectorsemit, for each measurement period, a spectrum which, in addition toinformation about the number of X-ray quanta of medium quantum energyreceived in each measurement period, and hence the intensity, alsocontains information about the respective quantum energy of the X-rayquanta, and thus the spectrum of the received X-ray radiation, whereinthe measurement result from the measurement object is calculated on thebasis of the information detected by the detectors relating to theintensity and quantum energy of the individual X-ray quanta in thereceived X-ray radiation, taking into account the intensity and thespectrum of the X-ray radiation emitted from a radiation source, whereinthe detectors include a plurality of parallel-connected comparators,each having a threshold value and each including an associated counter,the comparators being designed to increment the respectively associatedcounter by one unit when the quantum energy of an X-ray quantum in thereceived X-ray radiation exceeds the threshold value of the respectivecomparator, and wherein the detectors include a plurality of pulse logicdevices, one pulse logic device being connected downstream from therespective comparators and upstream of the respective counters, thepulse logic devices providing time normalization of the output signalsfrom the comparators.